Tunable optical filters are commonly used in optical devices and systems where an optical spectrum of light needs to be modified, with some wavelengths passing through the filter and some wavelengths blocked by the filter. The wavelengths that are passed by the filter form a band of wavelengths called a transmission band. When the filter is tuned, a central wavelength of the transmission band of the filter is tuned. Tunable optical filters are used in optical communications systems to distinguish between optical signals at different wavelengths carrying different information channels. The optical signals at a single central wavelength are called wavelength channels.
A tunable optical filter is frequently used as a key component of an optical spectrometer. In a scanning optical spectrometer, the central wavelength of a tunable filter is scanned, while the detected optical signal is continuously measured. As a result, an optical spectrum of an optical signal is obtained.
Traditional spectrometers are manufactured as laboratory devices which operate under laboratory environmental conditions. A periodic wavelength and optical power calibration is required to ensure the wavelength and power accuracy of these devices. Traditional spectrometers are generally bulky and costly.
Optical communication systems employing wavelength division multiplexing (WDM) technology achieve large transmission capacity by spacing wavelengths of individual wavelength channels as closely as possible, typically less than a nanometer apart. As the channel spacing decreases, monitoring spectral characteristics of the wavelength channels becomes more important. For example, optical channel monitoring can be used to detect an undesirable wavelength drift of wavelength channels.
Optical communication systems employ optical channel monitors (OCM), which function similarly to laboratory grade spectrometers, but are environmentally robust, inexpensive, and compact in size. The spectral resolution and wavelength accuracy of an OCM must be nearly the same as those of a laboratory grade spectrometer, however without requiring extra calibration over the lifetime of the device.
It is preferable that an OCM is capable not only of monitoring all channels in one optical hand, but also monitoring an optical signal-to-noise ratio (OSNR) for each wavelength channel. Today's WDM networks may employ as many as 100 channels at approximately 0.4 nm spacing. The OSNR measurement calls for 0.2 nm spectral resolution or better, at a dynamic range of 40-50 dB.
OCM are used at wavelength routing network nodes to provide straightforward monitoring and alarm-condition recognition. Furthermore, OCM are used to provide “per-channel” optical power measurement for network control loops. Network control loops are thus limited in their response, at least in part, by the rate at which the OCM can update the optical power measurement.
Typically, a network node contains multiple monitoring locations. If there are N monitoring nodes, one brute force approach would be to deploy N OCM, associating one with each monitoring point. This scales the cost, space and power dissipation associated with the aggregate monitoring function by the same factor N. A more common approach is to deploy an N×1 selector switch at an input of a single OCM, considerably reducing the size and cost associated with multi-point monitoring. However, this approach suffers from poorer response time as the aggregate time to monitor all N points is equal totime=n×(switch settling time+OCM scan time)
An OCM is frequently used to control the attenuation of a wavelength selective switch (WSS) in a reconfigurable optical add/drop node. The optical signal is tapped both before and after the WSS, at each port of the WSS, for the purpose of monitoring. Again, the brute force approach could be to use an independent OCM for each of these locations. Given some means to synchronize the scan of these multiple OCM, the channel power of the input and the output could be measured at one time, thereby giving the information required to calculate the attenuation for each measured channel. However, this approach suffers from aforementioned cost and size disadvantages of deploying multiple OCM. As mentioned previously, a more typical application is to implement a selector switch in front of the OCM to reduce size and cost. In this case, in addition to the considerably slower measurement time, the measurement at input and output ports are no longer made at the same time. To the extent that the power levels being measured are not strictly constant over time, there is some confounding imperfect measurement of the inferred WSS attenuation.
One type of industrial-grade OCM acquires all monitored spectral points of an optical spectrum of an input signal in parallel by dispersing the input light in space and using a photodiode array to simultaneously acquire spectral information at a plurality of monitored frequencies. Disadvantageously, the number of photodiodes in the photodiode array scales proportionally to the number of wavelength channels and spectral resolution, thereby increasing the size and cost of the device and reducing its reliability. If the OSNR of each channel is to be measured, several photodetectors have to be provided within the dispersed light of a single channel. Since current photodiode arrays at telecommunications wavelengths of 1.5 to 1.6 micrometers are often supplied in strips of up to 128 photodiodes, this allows monitoring of just over 30 channels. 512 element arrays are available, which is sufficient for monitoring about 100 channels. However, these arrays are expensive.
Another type of industrial-grade OCM acquires the spectrum by angle-tuning a dispersive element, for example a diffraction grating. U.S. Pat. No. 6,118,530 by Bouevitch et al. teaches a scanning frictionless spectrometer with magnetically actuated flexure-supported diffraction grating and a dedicated separate channel for accurate wavelength referencing during each scan. The advantage of a scanning approach is based on the ability to continuously sweep the wavelength, which greatly improves fidelity of spectra obtained, as well as accuracy of signal-to-noise and peak wavelengths measurements. Detrimentally, a scanning spectrometer is often slower than its detector array based counterpart. A slower measurement speed results from the fact that, in a conventional scanning spectrometer, most of incoming light is discarded, and only a narrow optical frequency component is allowed to impinge on a photodetector at any given time.
An intermediate approach, seeking to benefit from the advantages of both a scanning spectrometer and a spectrometer having a photodetector array, has been disclosed by Onishi et al. in US Patent Application Publication 2007/0177145. Referring to FIG. 1, a spectrometer 100 of Onishi et al. is presented in a simplified form. The spectrometer 100 includes an input optical fiber 102, a collimating lens 104, an acousto-optical deflector (AOD) 106 controlled by a controller 107, a diffraction grating 108, a focusing lens 110, photodetectors 112, 113, and 114, analog-to-digital converters (ADC) 115, 116, and 117, a signal processing unit 118, and a display 120. In operation, an input optical signal 101 exits the optical fiber 102 at its tip and is collimated by the collimating lens 104. A collimated optical beam 105 is deviated by the AOD 106 by a controlled variable angle, in dependence on the frequency of a control signal applied by the controller 107 to the AOD 106. The optical beam 105 is dispersed by the diffraction grating 108 into individual wavelengths. The photodetectors 112 to 114 each detect a fraction of a wavelength-dispersed optical beam 111. When the collimated optical beam 105 is angle-tuned by scanning the AOD control signal frequency, the optical spectrum of the wavelength-dispersed optical beam 111 is scanned across the photodetectors 112 to 114. Thus, only a fraction of the optical spectrum needs to be scanned across individual photodetectors 112 to 114. The ADC 115 to 117 digitize the detected signals and provide the digitized signals to the signal processing unit 118. The signal processing unit 118 then combines the individual fragments of the spectrum and displays the spectrum on the display 120.
One disadvantage of the spectrometer 100 is a reduced reliability, limited scanning range, and high cost. Another is large size, which is detrimental in OCM applications. Yet another disadvantage, which is common to all prior-art spectrometers disclosed above, is that it only has a single optical signal path. Thus, when multiple optical signals need to be measured, multiple OCM have to be used, which increases size and cost of the equipment. Alternatively, 1×N optical selector switch could be used as explained above, which considerably slows down the measurements.
The prior art is lacking an optical tunable filter/spectrometer that would be usable for optical channel monitoring applications while being robust, inexpensive, having a high spectral resolution and a large scanning range, while being capable of monitoring multiple optical signals without sacrificing the measuring time. Accordingly, it is a goal of the present invention to provide such a tunable filter/spectrometer.